Polycarbonates (“PC”) are synthetic thermoplastic resins derived from bisphenols and phosgene, or their derivatives. They are linear polyesters of carbonic acid and can be formed from dihydroxy compounds and carbonate diesters, or by ester interchange. Polymerization may be in aqueous, interfacial, or in nonaqueous solution.
Polycarbonate-based materials are used in a broad variety of applications because of their high transparency, clarity, heat resistance, ignition resistance, toughness, stability, impact resistance, creep resistance, and mechanical strength. However, polycarbonate-based materials are less resistant to scratching and surface marring compared to other materials. In particular, polycarbonates based on bisphenol A (“BPA”) have limited scratch resistance. One method of preventing or minimizing scratch damage is to apply a hardcoat to an article formed from a BPA polycarbonate. Such an approach has several disadvantages, including requiring another manufacturing step, thus adding additional cost to the article, compromised durability and increased production complexity. Another method is to use a scratch-resistant material made from a copolymer of BPA and dimethyl bisphenol cyclohexane (“DMBPC”). However, these copolymers also have decreased impact properties and ductility compared to polycarbonates based on BPA. In contrast, polymers such as poly(methyl methacrylate) (“PMMA”) possess a high resistance to scratching. However, PMMA does not possess the appropriate structural properties, e.g. it does not have the mechanical strength of PC and it behaves in a brittle manner under an impact force.
Scratch resistance is useful for articles whose exterior surface may be subject to physical contact by other objects. For example, everyday activities which can scratch an article can include sliding on a surface, dropping, and rubbing against other items such as coins or keys when placed in a pocket. Polymer compositions with scratch resistance are therefore desirable in articles requiring a durable surface finish and appearance. Frequently, it is desirable that these articles have not only the scratch resistance afforded by a PMMA polymer, but also the strength and clarity of a PC polymer.
Copolymers of PC and PMMA is potentially an attractive solution to the foregoing problem. Unfortunately, despite significant need and effort to combine the desirable properties of these materials, PC and PMMA, there has been limited success. For example, the synthesis of polycarbonate-poly(methyl methacrylate) copolymer via sonochemical polymerization of methyl methacrylate (“MMA”) monomers with PC has been reported (see M. Choi, et al., Macromol. Symp. (2007) 249-250:350-356). In the method described, PC radicals were generated via sonication and then free radical polymerization with MMA allowed to proceed. Despite the ability to generate PC-PMMA copolymers, the method did not yield high molecular weight copolymers nor does free radical polymerization provide control over the final architecture of the copolymer. E. A. Kang and co-workers (see Poly. Eng. Sci. (2006) 40:2374-2384) prepared PC-PMMA copolymers utilizing vinyl-terminated PC that was polymerized with MMA monomers in the presence of azobisisobutyronitrile (“AIBN”) as a free radical initiator. The resulting copolymers had a random morphology. U.S. Pat. No. 4,310,642 discloses the synthesis of graft copolymers of polycarbonate with ethylenic monomers via free radical polymerization. However, as discussed above, free radical polymerization provides little control over the final architecture of the copolymer. US2009/0142537 discloses a thermoplastic blend of PC and a copolymer of MMA and napthyl methacrylate or a substituted methacrylate. Although this blend does provide improved scratch resistance with maintenance of PC optical properties, it is a blend and does not have the advantages of block or graft copolymer. The superiority of a copolymer versus a blend in achieving scratch resistance was explored by M. Okamoto (see J. Appl. Poly. Sci. (2002) 83:2774-2779). Okamoto described the preparation of PC-PMMA graft copolymers from PC oligomers and PMMA macromonomers. The PC-PMMA copolymers of Okamoto had superior clarity and hardness compared to the PC/PMMA blends, but
It would be desirable to provide a polycarbonate composition having improved anti-scratch properties that retains the desirable properties of polycarbonate. In particular, it would be desirable to provide such polycarbonates possessing these properties without the need for additional coating or post-mold treatments. The polycarbonate composition would be useful for certain transparent articles, such as optical parts, among other applications.
Accordingly, it would be beneficial to provide block and graft copolymers of polycarbonate and poly(methyl methacrylate) that retain appropriate impact strength and optical clarity properties.